If you think only of the ingredients that are in it, I ate haggis while I was in Scotland. (Honestly, it didn't taste that bad, though.)
The most disgusting-tasting thing I have eaten, though, that would be a Japanese food called umeboshi. It's basically just a type of pickled plum, so I don't now why it tastes so unimaginably vile, but during our Japanese cooking class at school, Mrs. Watanabe pulled us aside one by one to have us try one of these awful things and filmed our reactions. And if Youtube is any indication, lots of others share this sentiment.
http://www.youtube.com/watch?v=KOsCfK1dC4I
http://www.youtube.com/watch?v=9-HnN3BGSnI